why does a furnace need outside air?

I was doing some insulation in the basement, and I noticed that the
previous owner had disconnected some ductwork (4") which connected to
the cold air return of the furnace. This ductwork lead to an outside
vent, and was intended to bring in outside air to the furnace.
I assume that since the builder went through the trouble to put that
in, that it's necessary.. but why is it necessary? Isn't there enough
air in the house for the furnace to use? The furnace has been running
for at least 4 years without it. I'm willing to repair this, but I'd
like to know the reasoning behind it. I've found out that vent is a
pretty major source of cold air coming into the basement.
Another bit of information: The house is very drafty, so there's no
concern about it being so sealed up that all the oxygen is gone. LOL

outside COLD air is more dense so it burns better, more energy
efficent.:)
Indoor air sometimes has pollutants like bleach that can rot out high
effceny furnace heat exchangers, this is a verified issue
burning indoor air means your pulling a vacuumn losing heated air and
dragging even more outdoor cold air indoors making your gas bill go up.
put the original vent arrangement back if you want your gas bill to be
lower

Giving the furnace its own direct source of outside air will make you
home less drafty. Really.
All furnaces (non-electric that is) use air to burn their fuel. That air
needs to be replaced. It will come from those cracks and holes all around
your home bringing cold air in and moving it around your home. If it has
its own air source it will mean less drafts.

--------------------
Are you talking about the cold air *return* via which the air heated
by the furnace returns to the furnace to be heated again.
Or are you talking about a duct to supply combustion air to the fire,
which then goes up the smoke stack or out the combustion air
discharge?

I'm with you the OP said "...the previous owner had disconnected some
ductwork (4") which connected to the cold air return of the
furnace...". That doesn't sound to me like it is being used for
combustion.

Yes and no. I have such a connection in my home. There is a 4" or so
duct that is piped into the return duct. That 4" pipe comes from
outside. So anytime hot air leavs the house through the roof, or the
bathroom fan, or furnace/hot water heater exhause, the cold air sucked
in is through the furnace. (in theory)
Its not directly used for combustion, but its for pressure equalization.
The air used for combustion which is exhausted is replaced by air coming
in from this pipe and into the return.

Boy, I'd disconnect that crap real fast. This is an energy disaster.
What makes you think outside air is only coming in to equalize pressure
when air is leaving via say a bathroom fan? In reality, the powrful
furnace blower is gonna draw outside air in all the time. And the
inside air will make it's way out, via things like the bathroom fan,
whether it's turned on or not, or any other ways to exit the house.
A normal house shouldn't need any air specifically brought in from the
outside when the furnace is heating/cooling. Housed get enough from
existing air leaks around doors, windows, opening/closing doors, etc.
If you have a high energy efficiency house that is sealed very tight
and you need more outside air, then an arrangement with a heat
exchanger is used. Anything else is just wasting a lot of energy.
IMO, the arrangement you have is similar to leaving a window partially
open.

You obviously never heard of Sick Building Syndrome! Indoor air is highly
polluted and needs a replenishment of fresh air. In the winter time the
house is usually shut up more than in the summer and contaminants just
build up. This is especially bad if someone has allergies to indoor
contaminants, a growing cause of asthma! Also the air exchanged with
opening a door is contributing unheated air to the house. The outdoor air
supply is being heated before entering the house.
http://www.epa.gov/iaq/pubs/sbs.html

I went through a building design construction course. Community
college level, once a week for 12 weeks if I remember.
On the furnace heating system picture it this way. Your house is an
insulated box with insulation for the outside walls only and on the
ceiling. The furnace fires up and the heated is recirculated
within this tightly insulated box to bring the air up to your set
(comfort) temperature.
Heated air will cool on its own mainly conduction loss with some loss
through leakage. The (heated) leakage air loss has to be replaced by
outside air. The best route for this outside air to enter the house
is to duct outside air into the return air duct so that it is heated
first before it joins the circulation.
It is very dangerous to over seal the house, ie make it too airtight,
and not have this outside fresh air intake. The reason is that heated
air normally rises up the chimney. But when the room air cools air
contracts and it will suck air (in the reverse direction) down the
chimney. This suction includes toxic burnt gases present in the
chimney and can be blown back into the house. You breath this stuff.
This is slow poisoning.
Another reason for the fresh air intake is that in the absence of one
the heated air inside the house will be drawn up the chimney together
with the furnace gasses. You lose expensive heated air up the chimney
every time the furnace fires up. The best solution is to have a
separate fresh air supply to the furnace burners. This way outside
air goes direct to the burners and goes up the chimney immediately
after combustion. The burners do not use up warmed room air.
Often all the contractors do is to have an open duct from the outside
to an opening near the burner assembly. Of course in winter this lets
in freezing cold air. Many homeowners plug this duct up with
fiberglass or remove the duct altogether. It will work but you will
be sending warmed room air up the chimney. I fabricated an enclosed
duct to trunk the duct air into the furnace. Cold air is drawn in
only when the furnace fires up, a per normal convection, and the cold
air never gets into the basement. I understand newer furnaces already
have this feature.
To recap there should be a fresh air duct from the outside and
connected to the furnace return air trunk. There should be a separate
fresh air duct to supply fresh air to the furnace burners.

I missed the most obvious danger. If the house is tightly sealed and
there is no fresh air intake from the outside the the only supply of
oxygen for the furnace is from the air already inside the house. You
will be burning up oxygen you need for breathing. Most houses are
somewhat "leaky" and some air will leak in. But you will still be
breathing air that has less oxygen and likely mixed with combusted air
containing CO2 and carbon monoxide.

Pressure equalization? Don't think so. That cold
air pipe is meant for a certain percentage of make
up air from the outside. Required in many
communities with newer homes that are built air
tight and is likely to be 10 to 15 percent of the
air. Seems like a bit much if in a cold place.
But the purpose is to supply fresh air because
much of the inside air is highly contaminated by
the materials used in building, furniture,
carpets, etc.

I'm sorry it took me so long to respond (Thanks for all the replies
from everyone)
It is a 4" circular ductwork that leads from the outside into the
regular "cold air return" (inside air source) from the furnace.

It is not combustion air. Air it is there to meet the fresh air
requirement.
______________________________
Keep the whole world singing . . . .
DanG (remove the sevens)
snipped-for-privacy@7cox.net
wrote:

Consult your local building permits office. Many
jurisdictions nowadays require an outside air duct
to supply air to non-electric furnaces. The building
code in Ontario, Canada, added this requiremennt
about 20 years ago (and made iit retroactive.)
If your house is appropriately insulated against
draughts, the outside air duct will be more efficient
and cheaper rather than less.

Outside air should be brought in through a fresh air exchanger. As Don said,
this is a requirement in new homes because of how well sealed new houses in
Ontario are. I don't think new houses in Ontario allow air in under the
walls like US houses do. After the walls are insulated, the walls are
covered with plastic, and caulking is applied around any boxes that protrude
through the plastic. I don't know if there's any kind of sealant or
insulation between the walls and the floor.
I live in the US and I put a fresh air exchanger and high efficiency air
filter to help deal with allergies. Our house smelled like paint for 6
months before we had this done. New carpet also puts out fumes. Another
issue with new homes is black soot in the carpet at the base of walls where
air from outside gets sucked into the house. I think the carpet acts as a
filter.
Randy R

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